If you don’t have Windows 10 Professional already installed, we recommend you first download and install it from here: https://www.microsoft.com/software-download/windows10
To Activate Windows 10 Professional with Retail Key follow this:
1.Click Start, Go to Settings
2.Select Update and Security
3.Click on Activation Tab located on the left side
4.Click on Activate or Change Product Key
5.Enter the License Key you received after purchase
6.Done, now your Windows is fully activated and you will have access to Premium features